The invention relates to a device for converting voltage, a traction network in an electric or hybrid vehicle, as well as a method for charging a battery in a traction network.
Traction networks in an electric or hybrid vehicle comprise a battery, an inverted rectifier and an electric machine that is normally designed as a three-phase electric machine. The inverted rectifier in this case comprises a DC link capacitor as well as three half bridges, wherein two transistors with freewheeling diodes are arranged in the half bridges. The transistors are actuated by a controller that controls the transistors such that the DC voltage is converted into three phase-shifted AC voltage signals, or conversely converts the three-phase AC voltage into a DC voltage in recuperative braking.
In order to achieve correspondingly extensive ranges, batteries with large energy densities are required. Short charging times necessitate high charging performances so that correspondingly high rated voltages of the batteries are sought. The currently available transistors have a limited electric strength, however, of about 600 V. Consequently, the potential battery rated voltages are restricted to about 400 V, and the corresponding external DC voltage sources are designed for 400 V.
A method for operating an inverted rectifier of a three-phase drive of an electric automobile as an onboard charger is known from EP 593 472 B1, wherein two bridge branches of this inverted rectifier are regulated as boost converters during a charging operation such that, depending on a chiefly sinusoidal, low-harmonic power supply with cos φ=1, a DC voltage is injected into a DC link capacitor of the inverted rectifier, and wherein another bridge branch of this inverted rectifier is regulated as a buck converter such that, depending on a charging characteristic of a traction battery of the electric automobile, a charging current from the DC link capacitor is injected into the traction battery. In an alternative embodiment, a bridge branch of this inverted rectifier is regulated as a boost converter during a charging operation such that, depending on a charging characteristic of a traction battery of the electric automobile, a charging current from the DC link capacitor is injected into the traction battery.
A charging system is known from DE 10 2009 033 185 B4 for charging a battery of a vehicle with a converter that is connected at its DC voltage side to a battery for converting the DC voltage supplied by a battery into a variable AC voltage that can be used for an electric motor to drive the vehicle. Moreover, the charging system has a switching unit that is connected to the AC voltage side of the converter and to the switching unit for connecting an external power supply network to the converter. In this case, the switching unit is provided between the converter and the electric motor to disconnect the connection before the converter is used as a charger for the battery, wherein a voltage adaptation module of the converter is provided to create a charging characteristic of the charging system according to the battery charging requirements. Moreover, the converter comprises a DC link whose voltage value is raised above a voltage value of the external power supply network, wherein the converter is designed such that no high currents flow to the external power supply network when plugging in the power grid charging connection. Moreover, the charging system comprises an enabling switching apparatus for releasing a fuel filler flap of the vehicle and for unlocking the at least one power grid charging connection when the voltage increasing apparatus has increased the voltage of the DC link to a higher voltage than the external power supply network.